Liquid crystal displays ("LCD's") and other devices in which the electro-optically active element comprises liquid crystal material are well known.
One type of device employs an encapsulated liquid crystal structure in which a liquid crystal composition is encapsulated or dispersed in a containment medium such as a polymer. When a voltage corresponding to a sufficiently strong electric field is applied via an electrode across the encapsulated liquid crystal structure (the "field-on" condition), the alignment of the liquid crystal molecules therein is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystal molecules is random and/or influenced by the liquid crystal-matrix interface, so that the structure scatters and/or absorbs incident light. The applied voltage at which the structure changes from its field-off condition to its field-on condition is generally referred to as the threshold voltage. Such devices can be used in displays, architectural partitions, automobile sun-roofs, privacy screens, and signs.
Among the displays which can be made are projection displays, in which the encapsulated liquid crystal structure forms an element which is used to control the projection of light onto a screen, such as an overhead projector for transparencies. Exemplary disclosures include Fergason, U.S. Pat. Nos. 4,613,207 (September 1986), 4,693,557 (September 1987), and 5,016,984 (May 1991); Williams et al., published PCT Application No. WO 90/05429 (May 1990); and Flasck, U.S. Pat. Nos. 5,022,750 (June 1991) and 5,024,524 (June 1991); the disclosures of which are incorporated herein by reference.
Naturally in such displays it is desirable to have as high a contrast ratio as possible. The scattering effect is particle size dependent, so that at the smaller median volume diameters at which the contrast ratio is useful, there is more scattering of light in the higher blue and green frequencies and less scattering in the shorter red frequencies, with the effect that the display appears unpleasantly reddish or brownish. This effect is most noticeable at intermediate voltages at which the display is not fully in the field-on or the field-off conditions, as for example when gray levels are being displayed, because off-neutral grays are more noticeable than an off-neutral black.
A related problem exists in colored displays, where separate elements comprising encapsulated liquid crystal structures are used to control light of different colors, typically three primary colors such as red, green, and blue. The contrast ratio for the green and the blue light controlling elements can be improved by optimizing the median volume diameter. But at such median volume diameters, the scattering effect for red light is diminished, so that the red light controlling element is a less efficient scatter compared to the blue and green ones, an undesirable result. One can increase the median volume diameter for the red light control element to increase its scattering efficiency, but this solution suffers from the drawback that the red contrast ratio is compromised. Also, it is an undesirable design complication to have differently constructed elements for the different colors. Another alternative is to increase the thickness of the encapsulated crystal layer in the red light control element, to increase the scattering of the red light, but this solution has its drawbacks, too. Increasing the thickness increases the threshold voltage, increasing the power requirements and making the device design undesirably complicated in that a different powering set-up is required for the red light element.
We have invented a display which overcomes the aforementioned limitations.